The present invention relates generally to optical filters utilizing fiber Bragg grating type structures, and mere particularly to an add-drop filter utilizing chiral elements capable of changing polarization angles of selected polarized signal components.
Add-drop filters have many industrial applicationsxe2x80x94for example in information processing, in telecommunication systems, and especially in optical fiber communication systems utilizing wavelength division multiplexing (WDM). An add-drop filter enables a new signal to be added at a particular wavelength xcexxe2x80x2k to a fiber optic line carrying n signal channels over a band of wavelengths encompassing xcex1 . . . xcexkxe2x88x921, xcexk, xcexk+1, . . . xcexn, while an existing signal, xcexk is simultaneously dropped from the signal group. This functionality is essential in telecommunication systems. Previously known add-drop filters are relatively complex devices that require several expensive components, such as polarization converters that not only increase the filter""s cost but also add to its complexity.
Presently used add-drop filters are mostly based on fiber Bragg Gratings (FBGs). FBGs are typically manufactured through irradiating an optical fiber made from a UV-sensitive material with UV light using a pre-designed phase mask. Another prior-art approach to manufacturing fiber Bragg gratings involves irradiating a UV-sensitive optical fiber with two interfering UV laser beams. However, the previously known FBGs are relatively expensive and difficult to manufacture. Some of the efficient modern add-drop filters provide better performance than older non-FBG filters, but because they utilize both FBGs and polarization converters, they are relatively expensive and complex and therefore difficult to fabricate.
It would thus be desirable to provide an add-drop filter utilizing a novel construction and inventive chiral elements that advantageously eliminates and replaces components required in previously known add-drop-filers while providing better performance and higher efficiency.